Chapter 4.1

Tips ⚠️️

  • Use try.purescript.org to test out the code examples in this book.
  • Whenever the example code starts with module Main where, make sure to clear out the code editor on try.purescript.org before pasting new code in. This will help to avoid unncessary errors

Function Declarations

Now that we've had the chance to play around with functions a bit, we'll take a closer look at how to create them.

When we learned how to create variables, I mentioned that the syntax to do this was:

name = value

The syntax for creating functions closely matches this, taking the form

name parameters = definition

That is, first we put the name of the function, then the names of its parameters, the = sign, and finally the actual calculation the function will perform.

For example:

module Main where
import Prelude

addFive num = num + 5

-- addFive is the function name
-- num (on the left side of the "=") is the parameter
-- num + 5 is the definition

We can name the parameters whatever we want; they use the same naming rules as variables (because function parameters are variables).

Next, lets try writing a function that multiplies its parameter by two. Paste the following into your code editor

module Main where
import Prelude

multByTwo theNumber = theNumber * 2

-- multByTwo is the function name
-- theNumber (on the left side of the "=") is the parameter
-- theNumber * 2 is the definition

Why not try a few on your own? Try writing the following functions:

  1. A function called addTen which adds 10 to its parameter
  2. A function called sub15 which subtracts 15 from its parameter
  3. A function called square which multiplies the parameter with itself
  4. A function called doNothing which simply returns the parameter without doing anything to it
Answers 
module Main where
import Prelude

addTen num = num + 10

sub15 param = param - 15

square x = x * x

doNothing num = num

-- For `doNothing`, you also could have done the following:
-- doNothing num = num * 1
-- doNothing num = num + 0

So, how'd you do? Sorry if #4 seemed tricky, but kudos to you if you got it right! Also, don't worry if your parameter names don't match mine; remember, you can name them whatever you want, as long as they make sense.

Invoking Functions

Creating functions is great, but we won't be able to enjoy them unless we actually use them. The syntax for using a function looks like this:

name parameters

The syntax to use them is nearly the same as creating them, we just omit the = and the definition. Lets compare the create vs usage syntax:

-- Function creation
addFive num = num + 5

-- Function Usage
addFive 10

Why did we use num during the creation but 10 during the usage?

As mentioned previously, num is a function parameter, which is essentially a placeholder. The function wants to add 5 to something, but it lets whoever uses the function pick what that number should be. The function basically says "If you want to use me, you need to tell me what num is first." So then, when we use the function, we tell it to use 10 for num.

However, we can't just simply throw addFive around willy-nilly; functions return values, and values have to be stored somewhere. Therefore, we'll need to use a variable to hold the result of using the addFive function:

module Main where
import Prelude

-- Function Declaration
addFive num = num + 5

-- Using our function
myVar :: Int
myVar = addFive 10

-- myVar = 15

We can use addFive more than once, and we can also use different values for the num parameter without any issues:

module Main where
import Prelude

-- Function Declaration
addFive num = num + 5

-- Using our function
myVar :: Int
myVar = addFive 10
-- myVar = 15

var2 :: Int
var2 = addFive 20
-- var2 = 25

var3 :: Int
var3 = addFive 0
-- var3 = 5

Lets try out a few more exercises.. In the above example:

  1. Add another variable called var4 and use addFive to set it to 6.
  2. Add another variable called var5 and use addFive to set it to 100.
Answer 
module Main where
import Prelude

module Main where
import Prelude

-- Function Declaration
addFive num = num + 5

-- Using our function
myVar :: Int
myVar = addFive 10
-- myVar = 15

var2 :: Int
var2 = addFive 20
-- var2 = 25

var3 :: Int
var3 = addFive 0
-- var3 = 5

var4 :: Int
var4 = addFive 1

var5 :: Int
var5 = addFive 95

If you got those right and everything is making sense, then nice work! If you're still feeling shaky on what's happening, you might want to go back through this chapter and the previous chapter from again. A second run through can do wonders to help solidify knowledge.

Summary

Functions are created using the syntax

name parameters = definition

Here's an example of a function that multiplies its parameter by 3

tripleNum num = num * 3

To use a function, we use the syntax:

name parameters

For example:

tripleNum 20

Values have to be stored in a variable, so because functions return values, we need to create a variable to store the result of our function. With that said, here's a full example of creating and using a function.

module Main where
import Prelude

-- Function Declaration
tripleNum num = num * 3

-- Using the function
myVar = tripleNum 20

Self Practice

Question 1.
Create a function that multiplies its parameter by 4, then create 2 variables that use the function

Question 2.
Using the code box below, create one variable using each of the functions

module Main where
import Prelude

addSeven num = num + 7

sub5 param = param - 5

square x = x * x

Question 3.
Create a function that cubes its parameter, then create 2 variables that use the function

Answers

Question 1.

module Main where
import Prelude

quadruple num = num * 4

money = quadruple 10

numberOfOnionRings = quadruple 100 -- Lets go!

Question 2.

module Main where
import Prelude

addSeven num = num + 7

sub5 param = param - 5

square x = x * x

-- Variables
num1 = addSeven 7

answer2 = sub5 30

anotherVar = square 10

Question 3.

module Main where
import Prelude

cubeTheNum y = y * y

-- Variables
answer1 = cubeTheNum 3

answer2 = cubeTheNum 10